Magnetic resonance imaging (MRI) is an indispensible modality in medical imaging primarily for its ability to distinguish among a wide array of human tissues as well as among their pathologies. There is an immense body of MRI methodologies at one's disposal to generate diagnostically meaningful contrast to target the medical conditions in question. In general, transverse magnetizations are generated by radiofrequency (RF) “excitation” in the imaged subject, and data acquisition measures the transverse magnetizations in the spatial frequency transform space (the “k-space”). The resultant image is a map of the transverse magnetizations in the imaged slice or volume.
The most common physical properties by which MRI distinguish tissue types and diseases are proton density, T1 (spin-lattice relaxation time), and T2 (spin-spin relaxation time). T2-weighted imaging, in particular, where image contrast is primarily based on differences in tissue T2 values, is essential in the evaluation of the cardiac and cardiovascular systems, the central nervous system, and the musculoskeletal system. It allows for visualization, for instance, of tissue edema, coronary artery patency, the distribution of cerebrospinal fluid and the synovial fluid.
T2-weighted imaging was originally achieved using the spin echo imaging, and later the turbo spin echo imaging, both of which are currently in clinical use. In the spin echo and its variants, transverse magnetizations are generated by excitation and are allowed to decay exponentially under the influence of T2. Imaging data acquisition executes at a desired time delay after excitation, known as the echo time (TE). In spin echo, the acquisition only samples a small portion of the k-space and is therefore short enough to be considered instantaneous compared to the T2 decay itself. In turbo spin echo, the acquisition is longer and it takes place during a significant portion of the T2 decay process. In such case, one acquires the most important portion of k-space (the center and near-center components) at TE.
T2 preparation, or “T2 Prep”, has been an alternative to spin echo-based imaging. Similar to spin echo, T2 Prep uses the radiofrequency manipulation of transverse magnetizations to enhance T2 contrast. Transverse magnetizations are generated and allowed to decay under T2, and refocusing pulses are used to prevent incoherence buildup among spins that resonate at different frequencies. However, in T2 Prep, special care is taken in the RF pulse design to reduce effects of imperfections in B0 (MR scanner longitudinal main field) and B1 (MR scanner transmitter RF field). Also, the transverse magnetizations are restored to the longitudinal configuration when the desired TE is reached, ready for use with any desired imaging acquisition that follows. (Usually a “spoiler” is required after longitudinal restoration to eliminate any residual left in the transverse configuration.)
FIG. 1 displays a typical and idealized T2 Prep module. An excitation RF pulse (typically a 90° pulse) is used to transfer magnetizations from the longitudinal configuration into the transverse configuration. The magnetizations start to experience T2 decay immediately. During the decay, a train of refocusing RF pulses (typically 180° pulses) repeatedly refocus the spins to undo the effects of off-resonance. During this time, transverse magnetizations of different T2's decay at different rates, establishing T2 contrast among them. Finally, a restoring RF pulse (typically a −90° or equivalent pulse) is used to return the magnetizations to its longitudinal configuration, carrying in their magnitude the desired T2-weighted contrast.
The configuration of a T2 Prep may vary in a few ways. The refocusing pulses of T2 Prep may be simple hard pulses, composite pulses (e.g. MLEV-weighted composite pulses (1-3)), or adiabatic pulses (4). The number of refocusing pulses is typically 2 or 4 in practice. The number is also typically even, to exploit B1 insensitivity. The 4-refocusing configuration is shown in FIG. 2.
In typical imaging applications, the duration of T2 Prep is set to be in the vicinity of the tissue T2s themselves—usually between 20 to 100 ms. After a T2 Prep module, the differences in transverse magnetizations among tissue types are maximized to reflect their range of T2 values. However such T2 contrast lasts only briefly before the signal is degraded by other sources (e.g. T1 relaxation). Thus, to reflect the most accurate T2-weighting, imaging data acquisition must start as early as possible after the end of the T2 Prep module. It is therefore very important to minimize the time delay between the end of the T2 Prep module and the onset of imaging data acquisition. In practice, however, a delay is often inevitable due to other preparations necessary before imaging (e.g. fat saturation, conventional motion tracking using pencil-beam navigator, discussed below). The imaging data acquisition must also finish before the T2 contrast degrades, in other words, within 1× or 2× tissue T2 after the T2 Prep module.
As a result, the acquisition window following each T2 Prep module is brief, with a duration similar to tissue T2 itself (20 to 100 ms). Usually one can only acquire a small portion of all necessary imaging data (a “segment” of k-space) due to a large variety of physiological and hardware limitations in acquisition speed. Hence, the T2 Prep-imaging tandem is executed repeatedly until all segments of the k-space are acquired, as shown in FIG. 3.
For T2-weighted cardiac imaging, in particular, a T2 Prep and the ensuing segment imaging are executed every one or two heartbeats, at a desired delay time after each cardiac synchronization trigger (usually the QRS complex of each heartbeat). FIG. 4 shows one repetition of such an arrangement. To track respiratory motion, the conventional pencil-beam navigator is executed, usually between T2 Prep and imaging. This causes a significant imaging delay during which the fresh T2 contrast degrades for a duration comparable to that of the imaging window itself. If any kind of motion data can be collected during the T2 Prep module, the delay time before imaging onset can be significantly reduced and the imaging window can be extended. This would present a significant saving to the number of segment repetitions required. If the order of T2 Prep and the respiratory pencil beam navigator were reversed, the time between T2 Prep and imaging could be reduced. However, this would degrade the quality of motion information obtained with the respiratory navigator, as the interval between the measurement of motion and actual imaging would increase significantly with possible detrimental effects for imaging.
In the standard design of T2 Prep, however, no imaging data or motion data is collected during T2 Prep. This amounts to a dead time comparable to the duration of the imaging window itself. The reason is, during T2 Prep, the transverse magnetizations of imaged tissue are interacting with RF pulses that disperse and refocus them. Measuring such magnetizations for imaging would not yield data consistent with the rest of the imaging data, leading to artifacts and unacceptable image quality.
MRI is a relatively slow imaging modality and motion of the sample or the subject causes significant image artifacts. In virtually all MRI scenarios, considerable efforts must be made to avoid motion in order to prevent severe corruption of image quality in the form of blurring, smearing, and ghosting. When motion is inevitable, it must be accurately tracked so that imaging data can be collected in synchrony with the motion cycle, or that the imaging data can be corrected for motion artifacts. Both cardiovascular and respiratory motion are relevant, though other types of motion exist and must also be addressed: voluntary and involuntary motion by the patient or subject, peristaltic motion in the gut, swallowing, head motion due to cardiac pulsatility, etc.
In the abdomen, where respiratory motion is problematic, T2-weighted imaging has been restricted to breath-hold imaging. Because respiratory motion in the abdomen is pervasive and nonrigid, complex measurements of the motion field is required if motion is to be corrected. This type of motion is very important when imaging organs in the abdominal cavity such as the liver, the kidneys, the pancreas, etc.
The heart is also imagined with T2-weighted MRI because myocardial edema can be assessed this way. It is a highly challenging region due to its constant motion, which is generally a mix of two parts: intrinsic cardiac motion and respiratory motion. To track these two types of motion, T2-weighted cardiac imaging borrows standard techniques from general cardiac imaging: for cardiac motion, additional electronics such as the electrocardiogram (ECG) can be used. To track respiratory motion, an external mechanical device known as the respiratory bellow can be used. As an alternative, the MR scanner may periodically acquire additional motion-tracking data known as the “navigator” to track respiratory position.
When T2 Prep is used in regions affected by respiratory motion, such as in cardiac T2-weighted imaging mentioned earlier, a “pencil beam navigator” is used as the standard method for tracking respiratory motion. The pencil beam navigator, also known as “NAV” or simply “the navigator,” is a dedicated process during which a 2D-selective excitation generates MR signal from a narrow column of tissues to indicate its 1D displacement. The column is usually placed across the lung-liver interface. Typically, each pencil-beam navigator lasts from 20 to 50 ms, and is executed between T2 Prep and imaging data acquisition. The delay before imaging window is significant, because its duration is comparable to the imaging window itself (1× or 2× tissue T2). This is a significant price to pay for the readout of a single point of respiratory position. Additionally, the use of the pencil-beam navigator also assumes respiratory motion to be 1D (namely in the foot-head direction), permitting only gating and 1D translational correction of respiratory motion. Though multiple navigators can be applied and used for more complex (e.g. affine) motion correction, the delay time due to multiple navigators can be prohibitive for maintaining T2 contrast.
Although during T2 Prep the transverse magnetizations are in flux and unsuitable for imaging, it is noteworthy that the magnetization is still suitable for motion tracking. This suggests that motion measurements can be performed during T2 Prep. Because motion tracking in MRI operates on much less information than image formation, it usually requires lower signal quality and less data. Researchers have reported numerous successful tracking techniques using limited data, including low resolution images, projections of the imaged slice, the average signal of the imaged slice, or some other small subsets of imaging data itself. If any of these limited-data motion measurements is merged into T2 Prep, motion can be tracked in 2 or more dimensions.
It is therefore desirable to provide a method of MRI imaging that allows for accurate acquisition of MRI in areas of the body with at least one source of motion without image artifacts.